The means of communication constituted by the electricity network, both low and medium voltage, is a means that was not initially conceived for the transmission of telecommunications signals. This means is an unscreened means in which the signal transmitted can radiate and interfere with other systems that are using the same frequencies. The case of amateur radio enthusiasts and other radio-communications services that include radio navigation, emergency services, etc. are relevant. In these services, a low interfering signal can interrupt the communication, for which reason it is particularly important to avoid power transmission at these frequencies.
It is because of this that the rules in force in many countries impose restrictions on power spectral density (PSD) that can be transmitted on certain frequencies, so that it is necessary to obtain deep notches for transmitting signals over the electricity network according to the rules. The process that is the object of the invention is designed to create notches of great depth with the maximum efficiency possible, covering the bandwidth strictly necessary.
This process seeks the adjustment of the signal transmitted to certain patterns of power imposed by the aforementioned legal reasons in an efficient manner. These power patterns tend to be defined as frequency ranges in which power transmission is strictly limited (for example, 30 dB less with respect to other frequencies) or even prohibited. Nevertheless, given the technological limitations it is not possible to obtain immediate transition between a frequency band with a particular power spectral density and another one with a different power spectral density, so it is not possible to abruptly eliminate the signal transmitted in a certain frequency range without reducing the power of the adjacent carriers to the notch. Thanks to the invention process this reduction can be performed in an efficient manner, by reducing the number of carriers adjacent to the notch affected by generating this notch to the maximum.
On the other hand, the windowing concepts and precisely the risen cosine window, are not inventions of this process, but are also known in the state of the art. For example, they can be found in texts on telecommunications such as “Digital Communications” by John G. Proakis from the McGraw-Hill publishing company, in “Discreet-Time Signal Processing” by Alan V. Oppenheim from the Prentice Hall publishing company and in “OFDM for Wireless Multimedia Communications” by Richard Van Nee and Ramjee Prasad from the “Artech House Publishers” company. In the first reference it is commented that the risen cosine window is used for the design of limited signals in the band (to avoid any interference between symbols), while in the last reference it is commented that the windowing process applied to the individual OFDM symbols is applied in the reduction of the spectrum of the signal outside the band.
The new aspect of the invention process lies in producing notches of more than 30 dB in a band in an efficient manner, by means of adjusting the power sent in certain carriers, the windowing of OFDM symbols plus the cyclic prefix and the use of suitable values for the number of active carriers and the size of the IDFT; something that cannot be deduced from the previous state of the art of this patent.